1. Field of the Invention
The present invention relates to an aluminum alloy clad material (brazing sheet) which is especially, although not exclusively, suitable for use as a drawn-cup type heat exchanger core plate and, more particularly, to an aluminum alloy clad material (brazing sheet) which not only has a high level of press formability and pitting corrosion resistance, but also is improved in brazability by preventing the erosion attack of a liquid brazing filler metal into a core material at a brazing temperature.
The present invention is also directed to a heat exchanger including tubes made of the above-mentioned aluminum alloy clad material having a high resistance to corrosion and a long service life.
2. Description of the Related Art
Heretofore, aluminum alloy heat exchangers have been employed in oil coolers, intercoolers and heaters for automobiles; evaporators and condensers for air conditioners; oil coolers for hydraulic equipment and industrial machines; or the like.
An aluminum alloy heat exchanger has been fabricated by stacking aluminum alloy clad sheets (brazing sheets), which have been formed to a desired configuration, so as to form fluid passages and securing corrugated aluminum alloy fins between the fluid passages by brazing. For example, in the fabrication of a drawn-cup type evaporator, core plates 1a and 1b made of a press-formed (stamped) aluminum alloy clad material (i.e., brazing sheet clad on both sides with a brazing filler material) and corrugated aluminum alloy fin material 2 are stacked as shown in FIGS. 1 and 2. The bonding of the core plates and fins thus stacked is achieved by melting the brazing filler metal of the core plates. In such a brazing procedure, coolant fluid passages 3 are formed between the core plates 1a and 1b.
The core plate has been constituted by a clad material (brazing sheet) in which Mn containing aluminum alloys, such as Al-Mn alloy, Al-Mn-Cu alloy, Al-Mn-Mg alloy, Al-Mn-Cu-Mg alloy, etc., more specifically, alloys designated by the Japanese Standard Association (JIS) as 3003 alloy, 3005 alloy and so forth, have been used as the core material and the core material has been clad on one or both sides thereof with a brazing filler material of an Al-Si alloy or other similar Al-Si system alloy, such as Al-Si-Mg alloy, Al-Si-Mg-Bi alloy, Al-Si-Mg-Be alloy, Al-Si-Bi alloy, Al-Si-Be alloy, Al-Si-Bi-Be or the like. An aluminum alloy, for example, Al-Mn alloy, Al-Mn-Mg alloy, Al-Mn-Cu alloy or Al-Mn-Zn alloy has been used as he fin material.
As a brazing method, vacuum brazing is typically used but flux brazing using a chloride flux, fluoride flux and so forth have been also employed.
As set forth above, in the brazing sheet heretofore used for the fabrication of an aluminum alloy core plate, Mn-containing aluminum alloys, such as 3003 alloy or 3005 alloy, have been used as the core material. However, when these known alloy materials are used in the coolant fluid passages, pitting corrosion occurs from the outer surface due to the inadequate pitting corrosion resistance of these materials, thereby causing perforation and leakage problems.
In an effort to improve the pitting corrosion resistance of the fluid passage materials, some fin materials, for example, Al-Mn-Zn alloy, Al-Mn-Sn alloy and so forth, which are electrochemically anodic (less noble) relative to the fluid passage material, have been studied with a view of preventing the corrosion of the fluid passages by the sacrificial anode effect of these fin materials. However, while the sacrificial anode effect of such fin materials is effectively obtained in the vicinity of the brazed joint sections of the fins and the fluid passages, in other places away from the joint sections, the sacrificial anode effect of the fins does not occur. Consequently, pitting corrosion occurs.
Further, during the brazing operation, a liquid brazing filler metal 4, 4 of the aluminum alloy clad material (brazing sheet), as shown in FIG. 6A, attacks or erodes the core material 5 (maximum erosion depth =c-d), as shown in FIG. 6B, and the amount of the brazing filler metal at the joint section becomes inadequate. As a result, the thickness (a-b) of the brazed fillet section, as shown in FIG. 5, is reduced and problems, such as reduction of the fillet strength or burst pressure of a heat exchanger or deterioration of the corrosion resistance in the eroded part of the core material, arise.
In order to eliminate such problems, the instant inventors have developed an aluminum alloy clad material for a drawn-cup type heat exchanger core plate in which the core material of the clad material is made of an Al-Mn aluminum alloy with the addition of Cu, Mg, Ti and Si and the Al-Mn alloy core material has a controlled relationship between the particle size and the amount of Mn compounds and has a periodically varying Ti content along its thickness. inventors' aluminum alloy clad material has been disclosed in Japanese Patent Application No. 1 - 76 776.
While a sufficient improvement in corrosion resistance is obtained in the previous inventors' clad material, there is the technical problem that cracking tends to occur in the core plate during the press-forming process.